Jerome M. Schulman

Ab initio heats of formation of medium-sized hydrocarbons: Part 14. Pyracylene and biphenylene

Abstract The structure and thermochemistry of pyracylene and biphenylene were studied by means of ab initio calculations at the 6-31G ∗ (SCF) and 6-31G ∗ (RMP2-FC)//6-31G ∗ levels. The heat of formation of pyracylene is calculated to be 103.4 and 103.0 kcal mol −1 at the SCF and RMP2 levels, respectively. For biphenylene, an isomer of acenaphthylene, the calculated Δ H 0 t is 99.2 and 99.8 kcal mol −1 at the SCF and RMP2 levels, in good agreement with an experimental value of 99.88 kcal mol −1 .

The N4 molecule and the N3 + ion

The possibility of the nitrogen molecule forming a stable dimer is investigated. Through ab initio calculations in an extended contracted-gaussian basis, it is demonstrated that N4 in tetrahedral geometry and N3 + in D 3h geometry are local minimum points on their respective internuclear potential energy surfaces. The N4 molecule is considerably higher in energy than two ground-state N2 molecules and some study of the barrier for N4 decomposition has been made. The bond lengths and force constants for the two systems are shown to be similar to their hydrocarbon counterparts, C4H4 and C3H3 +, tetrahedrane and cyclopropenyl cation. Other related systems are discussed as well.

On the structure and thermochemistry of [18]annulene

The structure and thermochemistry of [18] annulene were studied by means of ab initio calculations at the 6-31G(SCF),6-31G∗(SCF) and 6-31G(RMP2-FC)//6-31G levels. At the RMP2 level, [18]annulene is 11.4 and 15.2 kcal mol−1 more stable in D6h symmetry than in its D3 and D3h forms, respectively. The calculated D6h geometry is consistent with the structure determined by X-ray crystallography. The RMP2 ab initio energy of this structure furnishes a ΔHf⊖ value in good agreement with the experimental value and enables an estimate of the resonance energy to be made. The latter relies upon the ab initio 6-31G (RMP2-FC) energies of linear polyenes, which are also reported here.

Correlation effects in the ab initio thermochemistry of benzene

Abstract Ab initio calculations of the energy of benzene in three large basis sets demonstrate that there is only a very small basis-set dependence of its resonance energy beyond 6-31G* at the SCF level and to second order in electron correlation.

Borazane and borazapropellanes: The possibility of inverted tetrahedral boron and nitrogen

The consideration of propellane-like structures whose bridgehead atoms are boron and nitrogen rather than carbon permits a study of B-N bonding and the effects of inverted tetrahedral configurations at these atoms. We analyze these systems primarily by ab initio structure calculation and the atoms-in-molecules properties method. The borazane molecule and its hexamethyl derivative, whose B-N bonds incontrovertibly exist, serve as the starting point. It is found that boraza[3.3.3]propellane resembles hexamethylborazane in its bond length, dipole moment and BN bond critical point. In contrast boraza[2.2.2]propellane does not exist as such but rather has the open form, 1-aza-4-borabicyclo[2.2.2] ctane. Boraza[1.1.1]propellane is found to be kinetically stable in C3v symmetry; it has a short B-N bond length but no bond critical point of its electronic charge density on the B-N axis.

Molecular polarizability and vibrational Raman tensor elements of the H+2 molecule

Molecular polarizability and Raman matrix elements for the bound vibrational states of H+2 are calculated. The increase in the magnitude of these matrix elements with vibrational quantum number and the ’’structure’’ of the contributions from the intermediate vibronic continuum functions are explored.

Nonsymmetric Perturbation Studies of One‐Electron Diatomic Molecular Ions

The interaction of a hydrogenic atom with a bare nucleus is studied perturbatively through infinite order in a model problem. Convergence of the energy and wavefunction expansions are discussed in Rayleigh‐Schrodinger and Brillouin‐Wigner nonsymmetric perturbation theories.

On the time evolution of a born-adiabatic molecular state

Abstract Time-dependent perturbation theory is applied to the problem of a “Born-adiabatic molecule” evolving in time when subject to an external time-dependent perturbation. The treatment uses double perturbation theory within the Dirac variations-of-constants method to correct both for non-adiabaticity and external perturbations. In the case of resonance non-adiabatic corrections can become large while away from resonance the wavefunction can be a time-dependent adiabatic wavefunction.